The present invention relates to a system for suppressing discharge of evaporated fuel gas wherein the evaporated fuel gas generated in a fuel tank of an internal combustion engine is introduced into an intake passage to suppress the discharge of the evaporated fuel gas into the atmosphere.
In a vehicle such as a motor vehicle, an adsorption device for evaporated fuel gas such as a charcoal canister has been generally used in preventing pollution of the atmosphere, by which device the evaporated fuel gas generated in a fuel tank or a float chamber of a carburetor is once adsorbed to prevent the evaporated fuel gas from being discharged into the atmosphere The evaporated fuel gas thus adsorbed and held in the charcoal canister is introduced into an intake passage through an evaporated gas passage having ports opening in the intake passage of the engine during the operation thereof.
The above-described ports have been in general so arranged with respect to an intake pipe that one of the ports located upstream of a throttle valve is open when the throttle valve is in the fully closed state while the other port located downstream of the throttle valve is open when the throttle valve is opened to an angle equal to or greater than a predetermined relatively small angle. Thus, the evaporated fuel gas is not introduced into the intake passage when the throttle valve is in its fully closed state, because the upstream port is in communication with the atmosphere, while the evaporated fuel gas is introduced into the intake passage when the throttle valve is opened to an angle equal to or greater than the above-described predetermined angle, because the downstream port is in communication with the negative pressure in the intake pipe.
In Japanese Patent Laid-Open No. 57-52663, for example, there is disclosed a construction wherein a port is located downstream of a throttle valve and a valve is provided in an evaporated gas passage between a canister and the port for opening and closing the evaporated gas passage in such a manner that the valve closes the evaporated gas passage during a low load condition of the engine such as idling thereof thereby intercepting the introduction of the evaporated fuel gas into the intake passage, while the evaporated gas passage is opened when the engine is into a high load condition, thereby introducing the evaporated fuel gas into the intake passage.
In such a prior art device, however, since the control of the evaporated fuel gas is limited only to the manner whether or not it is introduced into the intake passage, the following problems arise:
First, a canister for adsorbing the evaporated fuel gas having a very large capacity is required, since no introduction of the evaporated fuel gas into the intake passage occurs very often under the low load conditions including idling of the engine. In addition, at the beginning of the introduction of the evaporated fuel gas, a very rich evaporated fuel gas is introduced into the intake passage so that the air/fuel ratio of the mixture supplied to the engine is made very high due to the introduction of the evaporated fuel gas, thereby deteriorating the emission of the exhaust gas and the drivability of the engine and causing stall of the engine at the worst. The flow rate of the evaporated fuel gas introduced through the port is uniquely determined by the cross-sectional area of the evaporated gas passage between the canister and the port, thereby requiring the capacity of the canister to be further enlarged. On the other hand, it is necessary to increase the amount of the evaporated fuel gas introduced into the intake passage in order to make the canister compact, thereby giving rise problems contradictory to that of the deterioration of the emission and the drivability.